Many conventional residential heating and cooling systems include a disposable air filter assembly which includes a frame, a sheet of flexible filter material, and a filter material support structure such as a mesh screen that supports the filter material within the frame. When such a filter becomes dirty or clogged, the entire filter (including the frame and support structure) is discarded, even though only the filter material is no longer functional. This is wasteful and adds cost to such filters. Also, because such filters are rather bulky, they are often purchased as needed rather than being stored for future use. Thus, changing a filter can require purchasing a new filter which is not always convenient so that filter replacement can be delayed resulting in reduced efficiency of the system.
One solution to these problems has been to provide filter assemblies that include reusable frames and replaceable filter media. U.S. Pat. No. 3,280,984 (Sexton et al.), U.S. Pat. No. 3,774,377 (Bishop), U.S. Pat. No. 3,789,589 (Delany et al.), U.S. Pat. No. 4,042,358 (Frohmader), U.S. Pat. No. 5,840,094 (Osendorf et al.), U.S. Pat. No. 6,033,453 (Weddell, III) and Japanese Unexamined patent Publication Hei 11-351657 published Dec. 24, 1999, provide illustrative examples of this approach. Also, U.S. patent application Ser. No. 10/172,575 filed Jun. 14, 2002, describes a filter assembly including a reusable frame and a filter element including a collapsible replaceable sheet of pleated filter media that can be collapsed for compact shipping, storage, and disposal, but which can be quickly and easily expanded and installed in the frame.
The sheet of filter media used in such filter assemblies is often graduated density media which increases in density from a portion of the media that defines its air inlet surface and stops larger particulates in the air, toward portions of more closed media which collect smaller particulates than are collected at the air inlet surface. It is important for maximum efficiency of such filter media that it is positioned in the filter assembly so that air will flow into the filter media through its air inlet surface.